Dark Energy Is Real, Galaxy Survey Shows

A survey of more than 200,000 galaxies made with the Anglo-Australian Telescope in eastern Australia has shown that "dark energy" is real and not a mistake in Einstein's conception of gravity.

The result is conveyed in two papers written by Dr. Chris Blake (Swinburne University of Technology, Melbourne, Australia) and colleagues, which have been accepted for publication in Monthly Notices of the Royal Astronomical Society.

The galaxy survey, called WiggleZ ("wiggles"), was set up to measure the properties of "dark energy", a concept invoked in the late 1990s to explain why the Universe seems to be expanding at an accelerating rate.

To account for the acceleration, astronomers had to either rewrite Einstein's theory of gravity or accept that the Universe is filled with a new kind of energy.

Dark energy was originally discovered by studying the brightness of distant supernovae -- exploding stars.

The WiggleZ project has used two other kinds of observations that provide an independent check on the supernovae results. One kind involves measuring a pattern in how galaxies are distributed in space ("baryon acoustic oscillations"), and the other, measuring how quickly clusters of galaxies have formed over time. [For details, see Notes for Editors, below.]

Both tests have confirmed the reality of dark energy.

WiggleZ is one of several baryon acoustic oscillations experiments planned or in progress, and is the first one at high redshift to bear significant fruit.

The survey mapped the distribution of galaxies in an unprecedented volume of the Universe, looking eight billion years back in time -- more than half the age of the Universe.

"This is the first individual galaxy survey to span such a long stretch of cosmic time," said Professor Michael Drinkwater (University of Queensland), one of the survey's leaders. "We've broken new ground."

Professor Matthew Colless is Director of the Australian Astronomical Observatory, which operates the Anglo-Australian Telescope, and a member of the WiggleZ team. "WiggleZ has been a success because we have an instrument attached to the telescope, a spectrograph, that is one of the best in the world for large galaxy surveys of this kind," he said.

The survey, which began in 2006 and finished this year, was led by Professor Warrick Couch of Swinburne University of Technology in Melbourne and Professor Michael Drinkwater of the University of Queensland. Dr. Chris Blake has led the analysis of the survey results.

# # #

The Australian Astronomical Observatory is Australia's national optical observatory, and is part of the Commonwealth Department of Innovation, Industry, Science and Research. It operates the 3.9-m Anglo-Australian Telescope and the 1.2-m UK Schmidt Telescope at Siding Spring Observatory in New South Wales, Australia.

One method of probing dark energy relies on detecting a pattern in how galaxies are distributed in space. Pairs of galaxies have a slight "preference" for being a certain distance apart. When measured in today's Universe, that distance is 490 million light-years.

Where does this pattern come from? It was created by pressure waves (sound waves) that existed in the Universe when it was very young -- no more than a few hundred thousand years old -- and very hot. As the Universe cooled, the pressure waves became "frozen" into its structure as regions of matter that were slightly more dense than other regions. The preferred positions of the galaxies are the points where the pressure was highest and thus matter more dense than at other places.

This preferred spacing of galaxies was originally detected in galaxies in the nearby Universe, not looking very far back in time. The overdensities of matter also show up in the Cosmic Microwave Background -- what we see when we look back to when the Universe was only 380,000 years old. At that time, the Universe was only about a thousandth the size it is today, and the pattern was correspondingly much smaller.

As the Universe has expanded over its lifetime, the galaxy clustering pattern has stretched accordingly. The WiggleZ team has measured the preferred spacing of galaxies when the Universe was about 8 billion years old (this corresponds to a redshift of 0.6; the universe is now 13.7 billion years old). WiggleZ is the first survey to detect the galaxy clustering pattern this far back in the Universe's history.

The WiggleZ measurement matches well with the preferred spacing predicted by the standard cosmological model incorporating dark energy. In this model, about 73% of the energy in the universe is in the form of dark energy and about 27% is in various forms of matter (mostly dark matter; only 4% is ordinary atomic matter).

2. Growth of galaxy clusters and superclusters

The second method of characterizing dark energy that WiggleZ used was to measure how it slows down the growth of galaxy clusters and superclusters.

Clusters and superclusters form when galaxies are pulled towards each other by their mutual gravity. But dark energy works against gravity, and should slow down the rate at which galaxies fall together into clusters.

If we can measure the velocity of infall at different times in the Universe's history, we can determine the effect of dark energy and see how it has changed over time. This would discriminate between different candidates for dark energy.

The observed redshift of a galaxy's spectrum is determined by both the overall expansion of the Universe and any additional motion it has, such as an infall velocity. These two factors can be teased apart, and so we can measure the growth rate for clusters and superclusters.

The WiggleZ team measured the rate at which such structures were growing at four periods in the Universe's history, spanning the last 8 billion years (the survey reaches out to a redshift of 1). These measurements matched well with predictions from the standard cosmological model incorporating dark energy.